Effects of BMP4/SMAD signaling pathway on mouse primordial follicle growth and survival via up-regulation of Sohlh2 and c-kit.

Bone morphogenetic protein 4 (BMP4) is essential for the development of primordial follicles, although its underlying mechanism remains largely unknown. By using cultured ovaries, the effects of BMP4 and the potential signal transduction pathways were investigated. Ovaries from 3-day-old female mouse pups were maintained in organ culture in the absence (control) or presence of BMP4 (100 ng/ml). At different culture time, the effects of BMP4 on primordial follicle growth and survival were assayed by follicle count and TUNEL labeling. The expression of phospho-SMAD1/5/8, Sohlh2, and c-kit were measured by immunohistochemistry, RT-PCR, and Western blotting. Immunohistochemistry was also performed to determine the expression pattern of BMP4, pSMAD1/5/8, Sohlh2, and c-kit in vivo during ovarian development. The results showed treatments of ovaries with BMP4 resulted in a significant (P < 0.05) increase on the primordial-to-primary follicle transition. The oocytes of primordial follicles treated with BMP4 were also less likely to undergo apoptosis. BMP4 enhanced the phosphorylation of SMAD1/5/8 and up-regulated the expression of Sohlh2 and c-kit in primordial follicles. During ovarian development in vivo, Sohlh2, and c-kit exhibited similar expression patterns to BMP4 and pSMAD1/5/8 in primordial follicles. The present studies suggest that BMP4/SMAD signaling pathway initiate primordial follicle growth and prevented oocyte apoptosis via up-regulation of Sohlh2 and c-kit.